1. Field of the Invention
This invention relates to deriving a representation of an ac waveform from two electrical signals in quadrature utilizing a digital computer.
2. Background Information
Electro-optical systems for measuring electric voltages are known. For instance, devices known as Pockel cells utilize certain crystals which exhibit birefringence, that is a difference in the index of refraction in two orthogonal planes, in the presence of an electric field. Some of these crystals, such as, for example, KDP (potassium dihydrogen phosphate), have a fourfold axis of rotary inversion. Such materials have the property that in the absence of an electric field the index of refraction for light propagating along the fourfold axis is independent of the plane of polarization of the light. However, if an electric field is applied parallel to the direction of the light, the index of refraction for light polarized in one direction transverse to the fourfold axis, known as the fast axis increases and that in an orthogonal direction, also transverse to the fourfold axis, and known as the slow axis, decreases by an amount which is proportional to the strength of the electric field. In such Pockel cell devices, if light is polarized in a plane which forms an angle to these transverse axes, the component of the polarized light in the direction of the slow axis with the decreased index of refraction is retarded with respect to the other component. If the crystal is aligned with its fourfold axis extending between the objects between which the voltage is to be measured, and the polarized light is directed parallel to the fourfold axis, the total retardation will be proportional to the total voltage differential between the two objects. This retardation is typically measured in wavelengths. The retardation is detected in an analyzer and converted to an electrical signal for producing an output representative of the magnitude of the voltage generating the field. Due to the cylic nature of this electrical signal, the output is only unambiguous for voltages producing a retardation which is less than the half wave voltage for the crystal. In KDP, this half wave voltage is about 11300 volts. This type of device is therefore not suitable for measuring transmission line voltages which can be 100,000 volts rms and more.
Other types of crystals used in Pockel cells respond to an electric field in a direction perpendicular to the direction of propagation of light through the cell. Such cells only provide an indication of the potential at the intersection of the beam with the field. Thus, a single cell cannot integrate the potential over the full space between two objects, and therefore these devices do not provide an accurate measurement of the voltage between the two objects. Systems using this type of Pockel cell commonly either, (1) measure the potential at one point and assume that the potential at all other points between the two objects can be derived from this single measurement, or (2) provide some sort of voltage divider and apply a fixed fraction of the line voltage to the cell in an arrangement which maintains the field within the cell constant. The problem with the first approach is that except for low impedance paths, the field along a path is sensitive to the location of any conducting or dielectric bodies in the vicinity of the path. Thus, if this type of Pockel cell is mounted on the surface of a conductor and the field measured, the reading would depend on the size and shape of the conductor, on the distance from the conductor to ground, on the location and potential of any nearby conductors, on the location of any insulating or conducting bodies near the sensor or on the ground beneath the sensor, and on the presence of any birds, rain droplets or snow between the sensor and ground. Thus, only under very ideal circumstances would accurate measurements be possible with such a system. The problem with the second approach is in providing an accurate stable voltage divider.
Optical voltage measuring systems are desirable because they provide good isolation from the voltage being measured. Through the use of optic fiber cables, it is possible to easily and conveniently provide remote indicators which are not subject to the electrical disturbances which remote indicators fed by electrical signals must contend with.
There remains, however, a need for an optical system for accurately measuring very large voltages such as, for example, those present in electrical transmission systems without the use of a voltage divider.
Subordinate to this need is a need for such an optical system which can integrate the field over the entire space between the objects, such as in the case of the electrical transmission system between line and ground.